Method of and system for recording and reproducing



y 3, 1934 w. K. GRIMWOOD 1,957,885

METHOD OF AND SYSTEM FOR RECORDING AND REPRODUCING Filed Feb. 10, 1952 [a5 R an -nu DEmDuLAToRI AMP LATOR oscumore sue nunPu: g9 AMP MODULATOR AMP OSCILLATOR nimflummt? 19 14 15 .75 12 Z gwuentozt Patented May 8 1934 UNITED STATES PATENT o FicE METHOD OF AND SYSTEM FOR RECORDING AND REPRODUCING Application February 10, 1932, Serial No. 592,085

11 Claims.

Ihis invention relates to the transmission of signals and more particularly to methods and apparatus for reducing the Width of the frequency band required for the transmission of signaling 5 current such for example as speech current.

The desirability of reducing the width of transmission bands for special purposes has long been recognized and is of prime importance in certain fields of communication and in recording and reproducing sound.

In its broad aspect the invention comprises a method and apparatus for reducing the width of a band of signaling frequencies for transmission by wire or radio or for recording and then reproducing the original signals by converting the reduced frequency band to its original form.

The method in general comprises modulating an audio frequency to a carrier of radio frequency, reducing the width of the frequency band in the radio frequency stage, and then supplying the frequency band of reduced width to the transmission network or demodulating and supplying the resulting audio frequency to a recording device. In reproduction the width of the frequency 5 band is increased while in a radio frequency stage, demodulated and fed through a suitable reproducing device.

Since the invention functions in the same manner for both direct communication and recording and reproducing it will be here described in detail only as applied to the recording and reproducing of sound.

The narrower the light slit used in recording sound on a film moving at a predetermined speed .35 the more accurate is the reproduction; but there is a limit to the size of the slit below which it is impractical to go due to the lower light intensity on the film and the consequent lower level of reproduction which results in distortion due to operating on the non-linear portion of the film characteristic curve.

It is an object of this invention to provide a sound recording and reproducing system in which increased accuracy in reproduction is gained with the use of a light slit of a given width.

Another object of the invention is to provide a :method for reducing the width of the frequency band necessary for the transmission or recording of sound without impairing the quality of the sound.

These and other objects may be accomplished, briefly, by using the sound wave microphone current to modulate a high frequency carrier, isolating the high frequency side band, dividing the side band frequency by some multiple thereof,

selecting and demodulating the desired frequency, and then recording the demodulated frequency. This produces a sound record in which the physical lengths of the individual variations are increased and in reproduction a corresponding increase in accuracy will be gained. If no gain in accuracy is desired the rate of travel of the film may be reduced without loss of accuracy in reproduction. This latter is highly desirable in reducing commercial 35 mm. sound films to 16 mm. sound films which generally are projected at a lower speed than are commercial films. In reproduction the pick-up current is converted into a high frequency side band from which is selected the harmonic corresponding to the original microphone current. This harmonic is then demodulated, amplified and reproduced as sound Waves in the usual manner.

The invention will be best understood from the following description of systems for carrying the invention into effect when read in connecton with the accompanying drawing in which:

Fig. 1 is a diagram showing a sound recording system including means for reducing the width of the frequency band;

Fig. 2 is a diagram showing a system suitable for reproducing the sound recorded by the system shown in Figure 1;

Figs. 3 and 4 are diagrams showing variational systems of those of Figures 1 and 2 respectively; and,

Fig. 5 is a diagram showing a suitable reproducing system where it is desired only'to halve the frequency band.

The several operations necessary in carrying out the present invention may be performed by operating units well known to those skilled in the art and are here shown conventionally for any detailed description thereof would serve only to lengthen it unnecessarily without aiding the understanding of the invention.

A step-down frequency converter is conventionally shown in Figure 1 as including a microphone 10 for transforming sound waves into electrical current. This current is amplified by a suitable amplifier 11 and fed to a suppressed carrier modulator 12 supplied with a radio frequency carrier by an oscillator 13. As is well known the output of the suppressed carrier modulator 12 is radio frequency sidebands corresponding to the audio frequency sound current input which includes both side bands. This output is fed through an amplifier 14 and then used to control the frequency of the Nth harmonic of a sub-multiple generator '15 having a fundamental frequency of 1/ N times the frequency of the oscillator 13. The fundamental frequency of the generator 15 is passed through an amplifier 16 which may be selective and fed to a demodulator 17 at which time the carrier frequency is resupplied by an oscillator 18. The audio frequency output of the demodulator is then amplified by an amplifier 19 and used to control a recording lamp 20. The light from the lamp 20 is concentrated on a moving film 21 by means of a slit 22 and a lens 23 to form a record 24 thereon of the divided frequency. In the above system a band of fre quencies from 1 to N in the microphone current is reduced to a band of frequency from l/N to M/ N at the recorder, where N is the number of the harmonic of the sub-multiple generator 15 controlled by the side band frequency supplied thereto. It will be obvious that the individual opacity variations of the record 24 will be increased in physical length over that they would be in usual recording. Also, if desired, the film 21 may be moved at a lower speed than in usual recording and still form a record having equal accuracy, i. e. the physical length of the opacity variations would be the same as those of a record as usually recorded.

The record 2 f formed on the film 21 as above described may be reproduced by the arrangement shown in Figure 2 in which a photoelectric, or other light sensitive cell is activated in a well known manner (not shown) with a light beam modulated by the record 24. The resulting current variations are fed through an amplifier 31 to a suppressed carrier modulator 32 supplied with a radio frequency carrier by an oscillator 33. A filter 34 selects the Nth harmonic of the output of the modulator 32 and supplies it to a demodulator 35 where the carrier is resupplied by an oscillator 36. The demodulated audio frequency output is passed through an amplifier 37 to a speaker unit 38 where it is audibly reproduced. Since the original sound Wave frequency corre sponded to the Nth harmonic of the fundamental i frequency used in recording and since the frequency used in reproduction corresponds to the Nth harmonic of the frequency set up by the photo electric cell, it is obvious that the sound waves reproduced by the speaker unit 38 are of the same frequency as the original sound waves impinging on the microphone 10.

The general operation of the complete system already described may briefly be stated as follows:

The sound wave current produced by the microphone is translated into radio frequency sidebands, divided by some multiple thereof, demodulated and the resulting audio frequency recorded upon a photographic film. In reproduction the sound wave current produced from the record is translated into radio frequency sidebands, multiplied by the same multiple used in dividing, demodulated and the resulting audio frequency current reproduced as sound waves.

The function of the sub-multiple generator 15 is to supply a frequency which is at all times related to the frequency applied to it by a factor 1/N where N is any integer. Consequently, the output of the sub-multiple generator is a Wave of l/N times the frequency of the input wave and this output wave contains all the elements of the signal present in the input wave. This output, when demodulated, gives an audio frequency band which has been multiplied by the factor 1 /N and yet contains all of the signal elements which were present in the original audio wave. For example,

if the original audio input covered the band ineluded betwen 1G0 and 10,000 cycles per second, this band, after modulation, multiplication, and demodulation, would be divided by the factor N and if the numerical value of the factor N is taken as 10, the resulting band would now be 10 to 1000 cycles per second. Such a narrow band is far easier to transmit or record than the wider original band and when modulated, multiplied, and demodulated at the receiving or reproducing station, such as is shown in Fig. 2, becomes a replica of the original audio frequency input.

If it is desired to employ the more common type of modulation in which the carrier is not suppressed, the arrangement illustrated in Figure 3 is suitable. In this arrangement the sound wave current coming from the amplifier 11 is supplied with a radio frequency carrier by the oscillatormodulator l2 and then, after amplification, used to control the Nth harmonic of the sub-multiple oscillator 15. The fundamental frequency of the oscillator 15 is amplified and passed through the demodulator 17 to the recording lamp 20. It is to be noted that the radio frequency impressed on the demodulator 17 still has its carrier and, therefore, an oscillator for resupplying the carrier, as was used in the arrangement of Figure 1, is unnecessary. If the system of Figure 2 is to be used for radio or carrier transmission the output of the amplifier 16 is fed to the transmission network.

A reproduction system employing the usual modulation method shown in Figure 4. In this arrangement the amplified current from the pickup device 30 is supplied with a radio frequency carrier the oscillator modulator 32. harmonic of the modulated carrier is selected by the amplifying filter 3i and fed to the demodulator 35, and then to the speaker unit'38. In this case, also, the carrier wave is present at the input of the demodulator 35 and no oscillator is needed 1 to resupply it, as was necessary in the arrangewas this arrangement the necessary frequency doubling is obtained by impressing the amplified output of the pick-up device 4:0 upon a full wave rectifier l2, the output of which is then passed through an amplifier 43 and utilized to activate I a speaker unit 44.

The full wave rectifier 42 may be replaced with a speaker unit of the telephone type from which the permanent magnet has been removed or omitted.

Although my invention has been described as applied to the'recording and reproducing of sound it will be obvious to those skilled in the art that it is readily adaptable to radio and wire communication systems as has be n indicated in connection with the description of the arrangement shown in Figure 3.

xcept for the special system shown in Figure 5 the dividing and multiplying of the frequency takes place at radio frequencies which makes it possible to carry out these operations with less distortion and loss of fidelity than is possible in dividing and multiplying an audio frequency.

It will be recognized that the illustrated systems specifically described herein comprise a series of operations each of which is well known and that therefore, the apparatus for carrying out the invention is susceptible of considerable modification and rearrangement without departing from the spirit of the invention, the scope of which is pointed out in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:-

1. The method of recording and reproducing sound that includes translating sound waves into electrical current waves, producing therefrom radio frequency sidebands corresponding thereto, dividing the sideband frequency by some multiple thereof, producing an audible frequency current wave corresponding to the divided frequency, making a record of said audio frequency current waves, reproducing from said record its recorded audio frequency wave, producing therefrom radio frequency sidebands corresponding thereto, multiplying the last mentioned sideband frequency by the same multiple used in dividing and producing therefrom an audio frequency current wave, and reproducing said last mentioned current wave as an audible sound wave.

2. The method of reducing the width of a band of signaling frequencies during transmission which comprises modulating the signal to a carrier frequency, suppressing the carrier leaving only the sidebands, dividing the sideband frequency by some multiple thereof, supplying a carrier to the divided sideband frequency and transmitting said divided sideband frequency, suppressing the carrier of the received sideband frequency, multiplying the received sideband frequency by the multiple originally used in frequency division, supplying a carrier to the multiplied sideband frequency, and demodulating the waves to normal signal frequency.

3. The method of reducing the width of a band of signaling frequencies during transmission which includes modulating the signal to a carrier frequency, producing therefrom a modulated wave of step-down frequency, transmitting said modulated wave of step-down frequency, translating the received modulated wave of step-down frequency to a higher frequency, and demodulating the wave to normal frequency.

4. The method of recording and reproducing sound which includes translating sound waves into electrical current frequencies, modulating the current frequencies to a carrier frequency, producing therefrom a modulated wave of stepdown frequency, demodulating the wave to audio frequency, making a record of the audio frequency wave, reproducing said record as an audio frequency wave, producing therefrom a modulated carrier wave of stepped-up frequency, demodulating the wave stepped-up in frequency and reproducing the demodulated wave as an audible sound wave.

5. The method of recording and reproducing sound which includes translating sound waves to electrical current frequencies, modulating the current frequency to a carrier frequency, reducing the frequency of the modulated carrier onehalf, making a record corresponding to the modulation of the reduced frequency carrier, reproducing from said record an audio frequency current wave and producing therefrom a rectified wave of doubled frequency, and reproducing said rectified wave as an audible sound wave.

6. In a sound recording and reproducing system the combination of means for the transforming of sound waves int-o electrical current waves, means for modulating the current wave to a carrier frequency and for suppressing all but the sidebands, means for dividing the frequency of said sidebands by a given number, means for supplying a carrier to the divided sidebands and for demodulating the modulated carrier, means for making a record of the current wave so produced, means for reproducing the current waves recorded on said record and for multiplying their frequency by the same number used in dividing, and means for reproducing sound waves from the multiplied frequency current waves so produced.

7. In a sound recording and reproducing system the combination of means for transforming sound waves into electrical current waves, means for modulating the current waves to a carrier frequency, means for dividing the frequency of the modulated carrier by a given number, means for demodulating the divided frequency carrier, recording means for making a record of the current waves so produced, means for reproducing the current waves recorded on said record, means for modulating the reproduced current waves to a carrier frequency and for multiplying the frequency of the last mentioned modulated carrier by the same number used in original frequency division, means for demodulating the multiplied frequency carrier. and means for reproducing sound waves from the multiplied frequency current so produced.

8. In a sound recording system, a recording unit, means for reducing the width of a band of audio frequency supplied to the recording unit comprising a source of sound current waves to be recorded, means for modulating the sound current waves to a carrier frequency, means for dividing the modulated carrier frequency and for demodulating the divided frequency.

9. In a transmission system, a transmitting station, means for reducing the width of a band of signaling frequencies comprising a source of signals to be transmitted, means for modulating the signals to a carrier frequency and for suppressing all but the sidebands, means for dividing the sideband frequencies by a given number and resupplying a carrier frequency, and means for demodulating and recording the resulting audio frequency.

10. The method of recording sound that includes translating sound waves into electrical current waves, supplying a carrier wave to said current waves and then suppressing all but the side band frequencies, multiplying the side band frequencies by the reciprocal of any desired integer, supplying the multiplied side band frequencies with a carrier frequency, removing all carrier frequencies therefrom and recording the remaining low frequency waves.

11. In the art of sound recording and reproducing, the method of changing the width of a band of sound current frequencies which comprises supplying the sound current frequencies with a carrier wave, suppressing the carrier wave leaving the side band frequencies, multiplying the side band frequencies by any suitable factor, supplying a carrier wave thereto and then demodulating to obtain a band of sound current frequencies of altered width.

WALTER. K. GRIMWOOD. 

